Frequency-domain interferometry has been widely adopted for performing a variety of measurements in a number of application areas. Examples include swept-source and spectral-domain implementations of optical coherence tomography (OCT) for noninvasive, depth-resolved imaging for a variety of biological and medical applications; optical frequency domain reflectometry (OFDR) for fiber optic sensing and testing of telecommunications networks, modules, and components; and frequency-modulated continuous-wave radar and lidar for remote sensing, detection, and ranging. Many such applications are based upon the ability of frequency domain interferometry to perform optical path length measurements. For both low-coherence and swept-wavelength implementations of frequency-domain interferometry, the resolution of the optical path length measurement is inversely proportional to the frequency bandwidth of the optical source. Axial resolutions on the order of 1 μm have been achieved with low-coherence approaches using extremely broadband supercontinuum sources. For SS-OCT, the axial resolution is typically limited to the order of 10 μm due to the more limited spectral breadth available from swept-wavelength sources.
Axial displacement sensitivities greatly exceeding the axial resolution of frequency-domain interferometry systems have been demonstrated by numerous groups using phase-sensitive techniques based on both low-coherence interferometry using spectrally dispersed detection as well as swept-wavelength interferometry. Both modes detect spectral interference fringes as a function of optical frequency and produce time-domain optical path length data by applying a Fourier transform to the acquired fringe patterns. Small displacements of discrete reflectors can be detected by noting changes in the phase of the complex-valued time-domain data at the location in the data array corresponding to the reflector. These phase measurements provide a relative displacement measurement from scan to scan, and have been applied to surface profiling, phase imaging, and Doppler flow measurements. Heretofore, however, the submicron displacements measured via phase have been relative to an arbitrary zero point within a single depth bin defined by the source-spectral-width-limited axial resolution of the system.